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  • 1
    ISSN: 1432-1106
    Keywords: Key words γ-Aminobutyric acid ; Muscimol ; Baclofen ; d.c. potential ; Extracellular pH ; Rat
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  Generally, increases in cortical activity go in parallel with negative shifts and decreases with positive shifts of cortical d.c. potentials. The aim of the present investigation was to test the effects of the inhibitory transmitter γ-aminobutyric acid (GABA) and of GABA receptor agonists on cortical d.c. potentials. Concomitant changes of local pH were measured to get first insights as to the mechanisms of the evoked d.c. changes. The experiments were carried out on anesthetized and artificially ventilated rats. d.c. potentials were recorded at a cortical depth of about 1000 μm by glass microelectrodes. Extracellular pH was measured by ion-selective microelectrodes. GABA (0.1 mol/l), the GABAA receptor agonist muscimol (0.1 mmol/l) and the GABAB receptor agonist baclofen (0.1 mmol/l) were microejected by pressure pulses at a distance of 20–40 μm from the recording electrode. GABA evoked positive d.c. shifts with low pressure ejection and long application times. With increasing pressure the positive d.c. shifts were initially superimposed by negative ones. The GABAA receptor agonist muscimol elicited negative and the GABAB receptor agonist baclofen positive displacements of the d.c. potential independent of application time or pressure. The negative d.c. shifts induced by GABA and muscimol were associated with an extracellular alkalization of up to 0.1 pH units. The findings led one to assume (1) that the negative d.c. shift after GABA application was due to a neuronal depolarization and to an increase in excitation via local alkalization and (2) that the positive d.c. shift mirrored neuronal hyperpolarization.
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  • 2
    ISSN: 1460-9568
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: The influence of follicular tissues on drug effects on ion channels in Xenopus oocytes was tested by investigating the pharmacological properties of a cloned potassium channel in oocytes with and without follicular tissues. The data show that the efficacy of blocking agents (ranging from metal ions to peptides) is drastically reduced by the follicular tissues (reductions by as much as 90% and increases of the IC50 values up to 30–fold). Furthermore, the time course of the blocking effect was slowed down by the tissues (increases of the t50 values up to 40–fold). The described impairment could be mitigated, but not abolished by partial removal of the follicular tissues (so-called defolliculation, leaving only the vitelline envelope and part of the follicle cells on the oocyte surface). The results indicate that the follicular tissues can induce significant errors in pharmacological measurements on membrane proteins in Xenopus oocytes.
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  • 3
    ISSN: 1460-9568
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Spreading depression (SD) is a well-known phenomenon in animal models of experimental epilepsy. However, the interaction of SD with epileptiform activity in human neuronal tissues is not clear. The aim of the present study was to investigate the effect of SD on synchronous rhythmic sharp field potentials in human neocortical slices. Spreading depression was elicited in human neocortical slices that exhibited sharp potentials. Extracellular field potentials were recorded from the third and fifth layers. SD significantly enhanced the repetition rate and amplitude of spontaneous rhythmic potentials in all tested slices. The results indicate that SD may facilitate the synchronization of different foci of rhythmic sharp field potentials and increase the excitability in human brain tissue.
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  • 4
    ISSN: 1432-0738
    Keywords: Key words n-Hexane  ;  Hexane metabolites  ;   Potassium current  ;  Oocyte
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract In order to study the mechanisms of acute n-hexane␣intoxication, the effects of n-hexane and its metabolites 2-hexanol, methyl-n-butyl ketone, 2,5-hexanediol and 2,5-hexanedione on the cloned voltage-operated potassium channels Kv1.1, Kv1.4, Kv2.1 and Kv3.4 were investigated with electrophysiological techniques in the expression system of Xenopus oocytes. n-Hexane had no effect at any channel, whereas some of its metabolites led to reductions of the potassium currents. The greatest effects obtained were caused by 2-hexanol at the Kv2.1 channel, resulting in reductions of 13% at 0␣mV with a concentration of 500 mg/l and IC50 of ca. 3500 mg/l. The reduction appeared to be caused by a shift of the current-voltage relation to the right. Methyl-n-butyl ketone showed smaller effects, whereas 2,5-hexanedione and 2,5-hexandiol were nearly ineffective. Concerning the different potassium channels, the sensitivity to the metabolites differed. The metabolites showed greatest sensitivity towards the Kv2.1 channel and lowest sensitivity towards the Kv3.4 channel. Since the n-hexane metabolite concentrations in the brain during acute n-hexane intoxication are unknown, the relevance of the data is still unclear. The size of the effects and the currently available data on tissue concentration, however, make it more likely that the action of n-hexane and its metabolites on voltage-operated potassium channels is not a major mechanism for acute neurotoxicity.
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  • 5
    ISSN: 1432-0738
    Keywords: Key words N-methyl-d-aspartate ; Glutamate receptor ; Glycol ether ; 2-Phenoxyethanol ; Ethylene glycol monophenyl ether
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The actions were examined of 17 frequently used glycol ether compounds on the glutamate receptor-mediated ion currents. The receptors were expressed in Xenopus oocytes by injection of rat brain mRNA. Most of the 17 glycol ethers exerted no effects on the glutamate subreceptors activated by kainate and N-methyl-d-aspartate (NMDA), whereas 2-phenoxyethanol (ethylene glycol monophenyl ether) caused a considerable reduction of NMDA-induced membrane currents in a reversible and concentration-dependent manner. The threshold concentration of the ethylene glycol monophenyl ether effect was 〈10 μmol/l. The concentration for a 50% inhibition (IC50) was ∼360 μmol/l. The results indicate a neurotoxic potential for 2-phenoxyethanol.
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  • 6
    ISSN: 1600-079X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: Recently, a species-dependent distribution of melatonin binding sites have been found in lamina I–V and lamina X of the spinal cord. In order to learn more about the function of spinal melatonin receptors, we investigated (i) the gene expression for melatonin receptor subtypes in lumbar and thoracal spinal cord tissue by means of the reverse-transcriptase polymerase chain reaction (RT-PCR) technique, and (ii) the electrophysiological and pharmacological properties of melatonin receptors heterologously expressed in Xenopus oocytes after injection of spinal cord mRNA by means of the voltage clamp technique. Because ample evidence indicates an antinociceptive effect of melatonin, (iii) the role of spinal melatonin receptors for maintaining mechanical and thermal hyperalgesia was studied in a rat model for postoperative pain. The RT-PCR data revealed that transcripts for MT1 and MT2 melatonin receptors are present in the dorsal and ventral horn of lumbar and thoracal spinal cord tissue. Injection of mRNA from lumbar spinal cord tissue into Xenopus oocytes led to the functional reconstitution of melatonin receptors which activate calcium-dependent chloride inward currents. Melatonin responses were abolished by simultaneous administration of the antagonists, 2-phenylmelatonin and luzindole and were unaffected by the MT2 antagonist 4-phenyl-2-propionamidotetralin. Intrathecal administration of different melatonin doses (10–100 nmol) did not inhibit mechanical or thermal hyperalgesia. However, intrathecal application of a low dose of morphine together with melatonin caused a brief antinociceptive effect suggesting an enhanced morphine analgesia by melatonin. In conclusion, the present study demonstrated for the first time the presence of transcripts of MT1 and MT2 receptors located in the dorsal and ventral horn of the spinal cord. Furthermore, spinal melatonin enhanced the antinociceptive effect of morphine indicating that melatonin acts as a neuromodulator in the spinal cord.
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  • 7
    ISSN: 1600-079X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Melatonin-sensitive receptors were expressed in Xenopus laevis oocytes following an injection of mRNA from rat brain. The administration of 0.1–100 μmol/L melatonin to voltage-clamped oocytes activates calcium-dependent chloride currents via a pertussis toxin-sensitive G protein and the phosphoinositol pathway. To determine which melatonin receptor type (mt1, MT2, MT3) is functionally expressed in the Xenopus oocytes, we used (i) agonists and antagonists of different receptor types to characterize the pharmacological profile of the expressed receptors and (ii) a strategy of inhibiting melatonin receptor function by antisense oligonucleotides. During pharmacological screening administration of the agonists 2-iodomelatonin and 2-iodo-N-butanoyl-5-methoxytryptamine (IbMT) to the oocytes resulted in oscillatory membrane currents, whereas the administration of the MT3 agonist 5-methoxycarbonylamino-N-acetyltryptamine (GR135,531) exerted no detectable membrane currents. The melatonin response was abolished by a preceding administration of the antagonists 2-phenylmelatonin and luzindole but was unaffected by the MT3 antagonist prazosin and the MT2 antagonist 4-phenyl-2-propionamidotetralin (4-P-PDOT). In the antisense experiments, in the control group the melatonin response occurred in 45 of 54 mRNA-injected oocytes (83%). Co-injection of the antisense oligonucleotide, corresponding to the mt1 receptor mRNA, caused a marked and significant reduction in the expression level (13%; P〈0.001). In conclusion, the results demonstrate that injection of mRNA from rat brain in Xenopus oocytes induced the expression of the mt1 receptor which is coupled to the phosphoinositol pathway.
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  • 8
    ISSN: 1432-0878
    Keywords: Key words: Epilepsy ; Dendrites ; Degenerative changes ; Endoplasmic reticulum ; Calcium-binding sites ; Helix pomatia (Mollusca)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract. The influence of epileptic activity on both the fine structure of neuronal processes and the subcellular distribution of calcium-binding sites was investigated in an epileptic model system, the buccal ganglion of Helix pomatia. Pentylenetetrazole was used to induce epileptic activity. Calcium-binding sites were visualized as electron-dense precipitates. Epileptic and control activity was intracellularly recorded from neuron B3 labeled with neurobiotin. After epileptic treatment, many processes contained vacuolated or electron-lucent areas next to morphologically intact areas. Most of these areas were enveloped by layers of endoplasmic reticulum. Lamellar formations of membranes occurred frequently. Calcium cytochemistry revealed a high content of dense precipitates within these formations of the endoplasmic reticulum. Local accumulations of diffuse precipitates were more frequent after epileptic activity than in controls. In contrast, structures such as lamellar bodies, cytosomes, and synapse-like formations, all of which contained many electron-dense precipitates, were apparently unchanged after epileptic activity. This study demonstrates that epileptic activity can lead to local degeneration of neuronal fibers and an associated increase in calcium-binding sites. It is suggested that calcium sequestration is locally increased within neuronal processes during epileptic activity.
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  • 9
    ISSN: 1460-9568
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: The effect of the epileptogenic agent pentylenetetrazol on eight cloned voltage-operated mammalian potassium channels (expressed in oocytes of Xenopus laevis) was investigated in order to contribute to an explanation for the brain area-specific differences in seizure susceptibility. Pentylenetetrazol increased the potassium currents at more negative and decreased them at more positive potentials for the channels of the Kvl gene family, whereas for the other channels the currents were decreased over the whole potential range. The sensitivities of the different potassium channels to the epileptogenic agent were different. At a potential of 0 mV, for example, there were strong reductions for the Kvl.1, Kvl.4 and Kv2.1 currents, whereas the decrease was smaller for the Kvl.3 and Kvl.6 currents and was negligible for the Kvl.2, Kvl.5 and Kv3.4 currents. Correlating these data with the distribution patterns of the potassium channels in the hippocampus, the neocortex and the cerebellum (representing examples of brain areas of distinct seizure susceptibility) revealed that in brain areas with higher seizure susceptibility the overall sensitivity of the potassium channels to the epileptogenic agent is augmented. As a whole, the findings give the first evidence that the differences in distributions and properties of potassium channels contribute to differences in the seizure susceptibility of brain areas.
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  • 10
    ISSN: 1432-2013
    Keywords: Key words Xenopus laevis ; Oocytes ; Mechanosensitivity ; Inositol trisphosphate ; Calcium ; Chloride current oscillations
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  During Ca2+-activated C– current oscillations a mechanical deformation of the Xenopus laevis oocyte by a fluid stream evokes transient inward currents of high amplitude (stream evoked inward current, I i,st). This current can be observed either in native or RNA-injected oocytes expressing ligand-controlled ion channels from rat brain. I i,st reversed at the equilibrium potential of chloride and was blocked by 9-anthracene carboxylic acid (2 mM). Power spectral analysis of the oscillations did not reveal a correlation between the features of the oscillations and the amplitude of I i,st. Antagonists of stretch-activated cation channels [gadolinium (100 µM) and lanthanum (1mM)] did not block I i,st. Calcium channel blockers [cobalt and manganese (10 mM)] did not inhibited I i,st and I i,st could also be elicited in calcium-free medium. Preloading oocytes with pertussis toxin (PTX) for 17 h prevented current oscillations and I i,st caffeine (10 mM), an antagonist of the liberation of calcium from intracellular stores, inhibited I i,st. Our results proride evidence for modulation of the mechanosensitivity of chloride currents by activation of intracellular second messenger cascades.
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